Robots have been proposed to clean and inspect vessels and underwater structures. Such robots typically include a drive subsystem for maneuvering the robot about the vessel or structure hull. Some drive subsystems include magnetic wheels or rollers. See U.S. Pat. Nos. 5,628,271; 3,088,429; and 2,104,062. The magnetic wheels of the '271 patent are stated to provide a holding power in excess of 2,000 pounds. The motor and drive train driving these wheels provides sufficient torque to overcome the strong magnetic tractive force. For example, the motor is stated to be a 24 volt DC motor providing 400 RPM.
Other drive subsystems include rollers and some means of adhering the robot to the hull via suction. See U.S. Pat. Nos. 4,809,383; 6,102,145; and 6,053,267. Some use rollers or wheels and a magnet spaced from the hull. See U.S. Pat. Nos. 6,564,815; 3,922,991; and 3,777,834. U.S. Pat. No. 4,697,537 discloses an impeller driven by a motor urging the robot against the hull.
Magnetic tracks and tracks with magnetic shoes have also been proposed. See U.S. Pat. Nos. 3,960,229; 2,132,661; 4,690,092; and 4,890,567 all disclosing electromagnets which may be selectively energized to control the drag force exerted by the magnets. U.S. Pat. No. 5,285,601, incorporated herein by this reference, discloses a blast cleaning device with two treads each including permanent magnets which continuously apply a tractive force.